Hydraulic cements principally containing calcium silicates are known as Portland cements. Portland cement concrete is among the most commonly used structural materials in the world. Annually about 600,000,000 tons of Portland cement are manufactured and utilized in the construction of all types of structures, including habitations, roadways, bridges, dams, etc.
General-purpose Portland cements consist essentially of 60-65% calcium-oxide, 20-24% silica, 4-8% alumina, and about 2-5% ferric oxide. Limestone, clay, shale, sandstone, bauxite and iron ore are among the raw materials used to produce a Portland cement product. Production of cement involves processing (burning) the raw material mixture at from about 2500.degree. to 2900.degree. F. (1400.degree.-1600.degree. C.), wherein the principal reactions in the heat treatment are as follows:
CaCo.sub.3 .fwdarw. CaO + Co.sub.2 PA0 4CaO+Al.sub.2 O.sub.3 +Fe.sub.2 O.sub.3 .fwdarw. 4CaO.sup..Al.sub.2 O.sub.3 .sup..Fe.sub.2 O.sub.3 (C.sub.4 AF) PA0 3caO+Al.sub.2 O.sub.3 .fwdarw. 3CaO.sup..Al.sub.2 O.sub.3 (C.sub.3 A) PA0 2caO+SiO.sub.2 .fwdarw. 2CaO.sup..SiO.sub.2 (C.sub.2 S) PA0 2caO.sup..SiO.sub.2 +CaO .fwdarw. 3CaO.sup..SiO.sub.2 (C.sub.3 S).
the expressions in parentheses indicate standard cement chemistry notations for the compounds formed on the right hand sides of the above equations. In this notation C=CaO, S=SiO.sub.2, A=Al.sub.2 O.sub.3, F=Fe.sub.2 O.sub.3, S=SO.sub.3, and H=H.sub.2 O. Such notation may be utilized hereinafter in discussing various compounds noted in the specification.
In the cement production process, the high temperature heat-treatment or burning, yields sintered or clinkered material which is then pulverized, normally with 4-5% gypsum, to produce the final cement product. The gypsum is added to the clinker in the final grinding process for the purpose of controlling the setting time of the cement when it is later mixed with water during the construction process. General-purpose Portland cement (usually designated ASTM Type I) contains typically about 50%, C.sub.3 S, 25% C.sub.2 S, 12% C.sub.3 A, 8% C.sub.4 AF, and 5% CS (gypsum). Thus in Portland cement, the total amount of calcium silicates is about 75%, and the predominant silicate is C.sub.3 S.
The clinker of such general purpose Portland cement is generally ground to a fineness of about 3,500-4,000 cm.sup.2 /g Blaine. The ground product is then bagged for warehousing and/or transportation to a concrete mixing plant. Such general-purpose Portland cements are required to meet ASTM standards, and exhibit 1800 psi and 2800 psi minimum compressive strength at 3 and 7 days, respectively, after hydration. The rate of strength (strength as a function of time) development is determined by standard ASTM procesure C109. The standard specification for Portland cement is covered by ASTM C150.
It will be appreciated that for many construction applications, the rate-of-strength development exhibited by Portland cements may be vitally important from an economic standpoint. Thus a Portland cement with a slow rate-of-strength development, or even a rate-of-strength development adequate to meet the ASTM specifications, will require support by forms during the period of time necessary to develop adequate strength to permit removal of the forms and undertake additional construction. Thus, lower rate-of-strength development slows construction, increases the inventory of forms, and increases labor costs. The economics of concrete construction is therefore directly affected by the rate-of-strength development. These strength development rates are inadequate for the precast, pre-stressed concrete industry products which often are required to acquire 3,000-4,000 psi strength at one day age.
In order to overcome the slow rate of strength development exhibited by general-purpose Portland cement, the industry has resorted to the production of high early strength Portland cement (ASTM type III), which consists of 55-70% C.sub.3 S, and is ground to 5,000-6,000 cm.sup.2 /g Blaine. Such cements, which obviously require more energy for their production, exhibit compressive strengths of the order of 2,000-2,500 psi at one day age. The minimum ASTM specification for compressive strength of type III Portland cement is 1,800 psi at one day and 2,800 psi at three days. However, for purposes of prestressed and some precast concrete products, even the strength development rate of normally cured Type III Portland cement concrete is not adequate. Therefore, steam curing must be employed in order to develop the required strengths in a shorter period of time.
In the chemistry and technology of Portland cements, the compounds C.sub.3 A and C.sub.4 AF are generally considered to be unimportant with regard to their strength contribution to the cements. In addition, the compound C.sub.2 S is capable of existing in two different crystalline forms, i.e., the gamma and the beta form. The gamma form is inert or non-hydraulic, while the beta form is slow to set and harden. The compound C.sub.3 S, however, is rapid hardening and a major contributor to the strength exhibited by conventional Portland cements. This is why when a high early strength Portland cement is desired, the current practice consists of modifying the compound composition of the cement by increasing the C.sub.3 S content as described above.
In any event, it will be appreciated that the development of Portland cement having strength development rates even higher than those presently available would be of great economic benefit.
It should be further considered that the U.S. Department of Commerce has classified the cement industry as one of the top 10 highest energy consuming industries. The total energy required to produce one ton of Portland cement is about 1300 kwh. About 100-150 kwh is required as direct electrical energy for the unit operations of crushing and grinding the raw materials and pulverizing the clinker to cement. A major part of the energy consumption is, however, in the form of heat which is needed for the heat-treatment, i.e., burning, of the raw materials. In order to achieve the necessary chemical reactions, the raw materials must be heated to temperatures in the neighborhood of 2,600.degree.-2,900.degree. F. Depending upon whether a dry or a wet process of cement making is used, the heat energy can vary from 800 to about 1,400 Kcals/Kg of clinker produced.
On the basis of 60-65% CaO present in a general-purpose Portland cement clinker, about 450-500 Kcals of the total heat required is spent for the formation of CaO from CaCO.sub.3. It is thus apparent that the heat required for the formation of calcium-oxide from calcium-carbonate is a major component of the total heat consumed in cement making. It is further apparent from their formulas, C.sub.3 S and C.sub.2 S, that tricalcium silicate requires the most CaO, and therefore the greatest consumption of energy for its production. Also, as noted from the high temperature necessary to effect the desired chemical reactions, it will be apparent that another significant component of the total heat consumed for clinker making is the heat lost by radiation from the high temperature zone of the cement kiln where the burning temperatures can be as high as 2,900.degree.-3000.degree. F. The heat lost by radiation is proportional to the fourth power of absolute temperatures.
From the above, it is apparent that from the standpoint of energy conservation, any reduction in the total amount of calcium-oxide in the cement product and/or reduction of the burning zone temperatures during the cement making process would lead to a considerable saving in energy.